LAUSR

Abstract Large and tiny Ophiomorpha nodosa Lundgren, 1891 and Thalassinoides suevicus Rieth, 1932, and the postlarval chamber Maiakarichnus currani, are described from the mid-Holocene Destacamento Rio Salado Member (northeastern Buenos Aires province, Argentina). Large specimens of O. nodosa consist of variably bifurcated horizontal galleries, connected in some cases by vertical shafts, and lined with pellets of different shape. Large T. suevicus are fewer and possess smooth outer walls. The smaller burrows of O. nodosa and T. suevicus are only a few centimeters from or even embedded in, the outer wall of the larger burrows, and are interpreted as the first settlement traces. A revised diagnosis of the spheroidal chamber M. currani, connected with O. nodosa and probably with T. suevicus, is submitted. The probable producer of these burrows and chambers is the ghost-shrimp Audacallichirus mirim. Morphological descriptions and burrowing behavior of A. mirim in modern and fossil populations from Argentina and Brazil suggest that this producer is represented at a higher level of ichnological abundance than previously recorded.